1. Field of the Invention
The present invention relates to a system of controlling the airflow output of a blower which is driven by a pole change motor and, particularly, to an airflow control system which causes less variation in the airflow when the number of poles of the driving motor is changed.
2. Description of the Prior Art
FIGS. 1(a) and (b) show the principle of the pole change motor, which comprises stator windings 1a, 1b, 2a, 2b, 3a, 3b, 4a, and 4b (shown diagrammatically for one phase) and rotor magnetic poles 5 (each pole shown by N or S). FIGS. 2(a) and 2(b) show the conventional pole change system, in which reference numeral 6 denotes stator windings including winding sections 61a, 61b, 62a, 62b, 63a, and 63b with terminals U1, U2, V1, V2, W1, and W2 associated thereto. Reference numerals 7, 8 and 9 denote switches, V.sub.R, V.sub.S and V.sub.T denote the voltages of 3-phase power lines R, S and T, O denotes the neutral point of the stator winding, 10 denotes a rotor of the pole change motor, 11 denotes a blower, 12 denotes a shaft connecting the motor 10 and blower 11, 14 denotes an airflow adjustment device in which a bar 14a is operated vertically to move a damper 14b so that the air passage resistance is varied, 13 denotes a device for producing an output controlling the position of the damper 14b, and 15 and 16 denote the inlet side and outlet side of the air passage, respectively.
FIG. 1(a) shows diagrammatically, the arrangement of a 4-pole motor, while FIG. 1(b) shows the motor converted into a 6-pole arrangement by reversing the currents in the windings 2b, 3a, 3b and 4a as shown by the dashed lines. Thus, the stator windings are partly connected differently so as to vary the current in the windings, thereby to accomplish a change in the number of poles of the motor. While FIGS. 1(a) and 1(b) show, as an example, the arrangement of changing the direction of current, it is also possible to exchange phase currents and their directions. In the arrangement of FIG. 2(b), the switch 7 is closed and switches 8 and 9 are kept open for operating the motor at a low speed, while the switch 7 is opened and switches 8 and 9 are closed so as to change the number of poles by varying the current for operating the motor at a high speed.
The operation of FIGS. 1(a), 1(b), 2(a) and 2(b) in the change of the number of poles is as follows. Considering the current in phase R, the winding 61b of FIG. 2(b) is located between the terminal U2 and the neutral point O, and the current direction is not changed by switching. The winding 61b corresponds to the windings 1a, 1b, 2a and 4b of FIGS. 1(a) and 1(b). The winding 61a is located between the terminals U1 and U2, causing the current direction to be changed by switching. The winding 61a corresponds to windings 2b, 3a, 3b and 4a in FIGS. 1(a) and 1(b).
The rotational speed n of the motor is given as: EQU n=120 f/P (RPM) (1)
where f represents the power frequency in Hertz, and P represents the number of poles of the motor. Accordingly, the motor speed can be varied by changing the number of poles. When the load of the motor varies, as in the case of a boiler blower which operates at a full load in the daytime and at a reduced load at night, the motor would be operated at a lower speed (increased number of poles) to meet a light load at night and at a high speed (decreased number of poles) to meet a heavy load in the daytime so as to minimize the total power consumption. The speed of the pole change motor is varied by changing the states of the switches 7, 8 and 9, and the output of the motor is conducted through the shaft 12 to the blower 11.
Referring now to FIG. 2(a), there are shown the airflow adjustment device 14 and the control signal producing device 13 provided for controlling the device 14. One input of a deviation detector 13c is supplied with an output voltage C from a boiler controller 13b. The output voltage C corresponds to the required airilow rate determined in accordance with various factors including the kind of the fuel used and the required power level. A second input of the deviation detector 13c receives signal X which is the value of the actual airflow to the boiler measured by airflow sensor 13a. The output of the deviation detector 13c, which is the difference between X and C, is converted to an output signal Y by an integrator 13d, the output Y being applied to a voltage-to-pressure converter 13e producing a variable pressure fed to the control device 14, which thus moves the damper 14b through the bar 14a in accordance with the control signal Y so as to control the amount of airflow. In FIG. 2(a), the output (Y), and the corresponding output from the control signal producing device 13 is varied until the actual airflow signal (X) becomes identical to the setting signal (C) by means of the deviation detector 13c and the integrator 13d, thereby to bring the airflow to a requisite set-value.
In the conventional airflow control system arranged as described above, when the motor speed is varied from high to low or from low to high, the correction of the airflow variation caused by the change in the motor speed is not in accord with the airflow variation caused by the damper 14b during the transient period because in usual cases, of the delay in the response time and the slow operation speed of the damper 14b. Therefore, if the blower 11 is used for a boiler, the combustion of the boiler could be extinguished or the internal pressure of the boiler could rise to a critical point of explosion, and the use of a pole change motor in such applications has not been appropriate.
The disadvantage in the prior art will be discussed below, using as an example a change-over operation from a low speed condition to a high speed condition. When the speed is changed from low to high, the revolution speed of the motor 10 is increased, with resulting increase of the airflow Q which is proportional to the motor speed. The increase in the airflow Q is reflected in an increase in the measured value X of the actual airflow which is compared with the desired airflow C in deviation detection 13c. The output Y from integrator 13d is represented by (Y=.intg.(C-X)dt) which means that the output Y is decreased by the increase in X, thereby causing the damper 14b to be moved toward its closed position to decrease the airflow. Due to the integration by integrator 13d and mechanical inertia, the response speed of the damper 14b is slower than that of the motor 10 and therefore there is a time delay between the increase in speed of the motor and the actual movement of the damper toward its closed position, so that an undesirable increase in airflow occurs at the time of the change-over.